Commit '28a0ce7 xtensa: use correct stack pointer for stack traces'changed the stack tracer from always reading the stack pointerregister to always using the saved value in the task descriptor.

The author was too dense to consider the fact that the saved stackvalue is stale for a running process und thus unusable for 'current'.

What we do now is to use the stack pointer register (a1) for when thetask is unknown - we can't help it then - or when the task is'current'. For everything else use the saved stack pointer valuecontained in the task descriptor.